Mesa-type photodiodes offer a number of advantages over planar avalanche photodiodes (APD), including reduced capacitance and increased bandwidth. However, mesa-type photodiodes suffer from poor reliability. This is due primarily to the structure which exposes the sensitive narrow bandgap absorption layer to foreign material such as air, SiN or other impurities.
Exposed narrow-bandgap photon absorption layer(s) on the etched sidewall of mesa-type PIN photodetectors imposes a great deal of reliability concern for almost all material systems, especially for InP/InGaAs PIN photodiodes (PDs) which are the primary candidates for long haul high data rate links. Passivation in the form of a non-conductive material coating is applied to seal the mesa walls, to give a stable, low dark current for reliable operation, and to form an insulating layer upon which to plate a bonding pad. Even though many efforts have been invested in the development of surface-passivation techniques to reduce the surface defects and traps, mesa-based PIN PDs have not yet been able to deliver satisfactory performance to pass the stringent Telcordia aging test.
However, in many cases, mesa-based PIN PD designs are the preferred configuration, such as for high-speed PIN PD arrays where a semi-insulating (S.I.) substrate is needed to reduce crosstalk noise between adjacent devices. Some higher speed applications also require mesa-based PIN PD designs to get higher bandwidth due to its lower parasitic capacitance.
The traditional methods of terminating the reliability-sensitive narrow-bandgap photon absorption layer(s) for PIN photodetectors are usually one of the following three categories. In a first method band gap layers are exposed to air only upon wafer sawing or cleaving into chips, i.e., no etched trench or mesa is formed within the chip area during the wafer processing. The reliability-sensitive narrow-bandgap photon absorption layer(s) extend to the edges of the chip without being etched, implanted, or diffused in any place within the chip area. The majority of the one-top-contact diffusion PINs are being made this way worldwide. The narrow-bandgap photon absorption layer(s) retains its integrity throughout the whole device area. A majority of the one-top-contact (no n-well) InP/InGaAs APDs are being made this way also, such as JDSU's U.S. Pat. No. 6,515,315. As for InAlAs/InGaAs APDs, there are two examples falling into this category. A first example is from Mitsubishi: OFC 2007 paper OThG2; PTL-18, p. 76 (2006); PTL-18, p. 1264 (2006); and Opt. Comm. 2005. Another example is from Multiplex: U.S. Pat. No. 7,105,369 and U.S. Pat. No. 6,756,613. But this method does not include mesa-type PDs.
In a second method, edge surfaces are exposed to air during trench or mesa etch but later the exposed surface(s) will be passivated by one or more of the following techniques: (a) epitaxial regrowth and (b) plasma enhanced chemical vapor deposition (PECVD) or sputtering dielectric film(s) such as SiNx or SiO2, or spin-on polyimide or benzocyclobutene (BCB) film. A few examples of prior art using epitaxial regrowth include the following patents: Opnext: U.S. Pat. No. 6,800,914; Mitsubishi: US patent application no. 2005/0025443 and US patent application no. 2005/0047743; TriQuint: U.S. Pat. No. 6,706,542; Sunitomo: U.S. Pat. No. 5,712,504; and HP: Journal of Quantum Electronics 34, p. 2321 (1998), U.S. Pat. Nos. 5,610,416, and 5,843,804, and 5,866,936. Dielectric or BCB/polyimide passivation is used for the majority of two-top-contact mesa PINs or APDs including laboratory designs and commercial products. Four examples for InAlAs/InGaAs APDs are from Picometrix: OFC 2005 paper OFM5; PTL-18, p. 1898 (2006); and US patent application no. 2004/0251483; Mitsubishi: U.S. Pat. Nos. 7,187,013, and 7,038,251; Hitachi: U.S. Pat. No. 5,543,629; and NEC: PTL-10, p. 576 (1998), PTL-8, p. 824 (1996), and PTL-3, 1115 (1991). The added steps for epitaxial regrowth add significant complexity and expense to photodiode manufacture. And dielectric coating or BCB alone has proven inconsistent in its ability to reduce dark current, and insufficient to meet data-com and telecom aging requirements.
A third method comprises passivation by ion implantation within the planar (mesa) surface or within the etched trench(es). Examples of prior art using this technique include: Mitsubishi: US patent application no. 2005/0224839, U.S. Pat. Nos. 7,038,251, 7,187,013, and US application no. 2005/0230706; Picometrix: US application no. 2004/0251483, and US application no. 2005/0156192; NEC: JLT-18, p. 2200 (2000); PTL-9, p. 1619 (1997); PTL-8, p. 827 (1996); and U.S. Pat. No. 6,229,162; and OCP: U.S. Pat. No. 6,753,214.
US application no. 2005/0224839 discloses an etched ring shaped trench surrounding the p-n junction with a Ti implant and diffused with Zn at the multiplication layer. This structure is for removing p-type characteristics and functions as a guard ring. U.S. Pat. No. 7,187,013 also requires an etched trench ring. Additional surface passivation is applied in the form of an AR coating over surfaces of the trench. US application no. 2005/0156192 rejects these previous designs. “An existing avalanche photodiode has an etched isolation ring which is etched down to expose the top of the high field avalanche region followed by a deep titanium implant to further isolated the high field region. This is then followed by a zinc diffusion to contact the p-type semiconductor region. This is a very complicated structure requiring critical etching and implant steps. In spite of these efforts, it is believed that the lifetime of this avalanche photodiode is ten times shorter than their standard planar avalanche photodiode and thus not sufficient for telecommunications use.” US application no. 2005/0156192 then discloses a passivated side region of a “mini-mesa” formed by wet oxidation and subsequent surface passivation of BCB, SiO2, SiN etc. But this type of passivation is only appropriate for Al containing material.
Among all these practices in the prior art, only diffusion-created field termination and surface passivation together can deliver satisfactory reliability performance to meet the data-com and telecom requirements. It is desired to find a process for this combination of passivation techniques without adding complexity and cost.
Accordingly, a process to create a reliable passivation of etched mesa-type PD surfaces without adding expensive additional processing steps remains highly desirable.
A mesa-type PD which can provide the reliability of planar PD is also highly desirable.